Experimental evidence that nest orientation influences microclimate in a temperate grassland

Abstract

Birds exhibit an assortment of behavioral strategies to cope with variable environmental conditions during reproduction, including altering nest construction behaviors. In species building enclosed domed nests, the microclimate within nests is influenced not only by its structure and the surrounding vegetation but also by the orientation of the nest opening. Many grassland-dependent birds build dome-shaped nests with clear directionality of openings. We studied two species in northeastern Kansas, United States that typically orient their nests east to northeast in this region. However, in a drought year, both Grasshopper Sparrows ( Ammodramus savannarum ) and Eastern Meadowlarks ( Sturnella magna ) shifted orientations of their nests southward toward prevailing winds. We hypothesized that this shift reduced the deleterious effects of heat stress on parents and developing young by diminishing morning solar radiation and increasing cooling due to the prevailing southerly winds. To test this hypothesis, we measured temperature, humidity, and wind speed at pairs of unoccupied, field-collected sparrow and meadowlark nests, experimentally placed to face south or east (control) in a non-drought year. Nest orientation affected the daily microclimate patterns, with south-facing nests warming later in the day relative to east-facing nests. The temperature differences depended upon humidity, with south-facing nests being relatively cooler under more humid conditions. This work provides the first experimental evidence of the benefits of plasticity in nest construction under challenging thermoregulatory conditions and shows how ground-nesting birds may reduce thermoregulatory demands during incubation under climate variation.

Keywords

• enclosed nest
• konza prairie
• nest architecture
• nest construction
• tallgrass prairie
• thermal tolerance